In recent years, the spread of cloud computing, cloud services, and the like make demand for improvement in the processing data rate and the data transmission rate, even greater. As optical transceivers to be used for data transmission, optical transceivers that realize the transmission rate of 100 Gbps or higher per channel are being introduced. The transmission rate over 100 Gbps is also realized by applying a DMT (Discrete Multi-Tone) scheme, which is used in the xDSL standards, to optical transmission networks.
Meanwhile, in modulation of high-speed optical transmission using a digital coherent technology, in order to prevent influence of chirping, external modulators such as a Mach-Zehnder (MZ) modulator are generally used. In an external modulator, the operating point drifts by temperature fluctuation, aging, and the like. As a method of compensating for the operating point drift in digital data transmission using an NRZ (Non-Return-to-Zero) scheme, a method has been known that superimposes a low-frequency signal on a drive signal of the optical modulator (see, for example, Patent document 1). Having superimposed with a low-frequency signal, the drive signal is amplitude-modulated gently between two levels of the logic values of input data, for example, electric potential levels corresponding to “0” level and “1” level. Then, by monitoring the output light from the optical modulator and controlling the bias voltage such that the superimposed low-frequency component is not detected, the operating point drift can be prevented.